In ordinary lenses other than the macro lens, a point of the reference in correcting aberration is located in infinitive far while macro lens compensate for aberration based upon the reference of proximal distance. Such a macro lens, however, is used considerably frequently for ordinary photographing, and it is desirable that the macro lens should fully cover an aberration correcting range from the ultimate proximal distance to the infinitive far.
A first embodiment of the prior art technology developed under these circumstances is a macro lens that employs a floating mechanism adapted to move at least two groups of lenses independent of each other during the focusing so as to cover a wide photographing range from the infinity to the equi-magnification (See Patent Document 1 and Patent Document 2 listed below, for example).
A second embodiment disclosed in the prior art technology is a macro lens that features a longer backfocus than its focal length by virtue of the first group of lenses of a retrofocusing type (See Patent Document 3 and Patent Document 4 listed below). Commercially available examples of this type include Pentax FA Macro 50 mm, F 2.8 from Asahi Optical Co., Ltd. (currently renamed as Pentax Corporation).
A third embodiment of the prior art technology currently commercially available is a reduced focal length macro lens capable of close-up shooting of an object in the proximal range but not in the range of infinity. For instance, available are AF Macro Zoom 3×-1×, F 1.7-2.8 from Konica Minolta, Inc., and Zuiko 20 mm, F 2 from Olympus Corp.
List of Cited Patent Documents:
Document 1                Japanese Provisional Patent Publication No. SHO62-195617        
Document 2                Japanese Provisional Patent Publication No. SHO63-247713        
Document 3                Japanese Provisional Patent Publication No. SHO62-160412        
Document 4                Japanese Provisional Patent Publication No. H01-214812        
The 35 mm film photographing lens is currently used not only for 35 mm film but also for optical apparatuses such as a digital still camera which charge coupled devices or CCDs dimensioned as small as 23.7 mm×15.6 mm are built in. In creating a picture this small, a focal length must be reduced so as to make the same angle of view as that of the photographing lens for the 35 mm film.
However, the macro lens utilizing the floating mechanism of the aforementioned first embodiment of the prior art is designed based upon a Gaussian lens, and as a consequence, the backfocus cannot be longer than the focal length that can be hardly shorter than about 50 mm. Hence, this type of macro lens has not been implemented as real products.
The second prior art embodiment is of retrofocusing type, and therefore, the lens structure is devised so as to cover a wide photographing range from the infinity to the equi-magnification, and especially, to minimize a variation in aberration. For that purpose, radii of curvature of the third group of lenses, r5 and r6, are not so different from each other, and a Z value indicating the norm for the centering of the lenses is small. As a result, precision in machining and accuracy of eccentricity greatly affect the entire lens system, and use of components manufactured with sufficient precision and accuracy brings about an adverse increase in the manufacturing cost.
The third prior art embodiment is a macro lens of the reduced focal length serving as a photographing lens only for close-up shooting of the object in the proximal range, which means the use of the lens is limited to this specific purpose. This macro lens is highly disadvantageous in that it is not suitable to take a picture of the object in the infinity range and does not meet the most common requirement.
The present invention is made to overcome the aforementioned disadvantaged in the macro lenses of the prior art, and accordingly, it is an object of the present invention to provide the one that takes a retrofocusing function for the purpose of downsizing, employs a floating mechanism to cover a wide photographing range from the equi-magnification to the infinity, and especially, attains a great imaging performance by appropriately compensating for spherical aberration, comatic aberration, and chromatic aberration of magnification.